Both in their number and per-incident quantity of spilled cargo, accidents attending shipment of liquid cargo by vessel have amply demonstrated the need for a way to at least reduce the quantity of spilled, potentially-polluting cargo. The most well-known recent accident involved the EXXON VALDEZ, a single hull vessel carrying crude oil.
The United States Coast Guard is understood to have estimated that had the EXXON VALDEZ had a double hull, 25% to 60% less oil would have been spilled. Others estimated the reduction in spilled quantity might have been as high as 75%.
Under United States law, double hulls are now required for certain types of new, cargo-carrying vessels. There are two common ways of "doubling" a ship's hull, namely, with a double bottom (protective in case of grounding) and with a double hull (double sides and double bottom--protective in case of collision or grounding).
But such "double" construction is extremely expensive, either in new vessel construction or in retrofit. However, it can be very effective. Some time ago, the United States Coast Guard studied 30 groundings occurring between 1969 and 1973. It concluded that a space between double bottoms of two meters (more than 6 feet) would have been effective in preventing inner bottom penetration with a 96% probability. But, of course, the loss in cargo space would be enormous.
Transportation of ballast, often sea water carried for stability when the vessel is otherwise empty, is also a consideration, at least in oil-carrying vessels. It is understood that oil tankers are required to have segregated ballast tanks which carry only sea water ballast and may not be used for cargo. Ballast tanks can comprise up to about one-third of the vessel's total capacity. Clearly, payload could be increased dramatically if space otherwise dedicated only to ballast could be used for "double duty" carrying.
And there is no prohibition against configuring a cargo vessel with spill-limiting systems which improve upon the performance of even a double hull vessel. But prior workers in this field have not been notably successful in configuring practical, highly-efficient and cost-effective systems.
For example, U.S. Pat. No. 334,481 (Sone) shows a vessel with rectilinear compartments transporting rigid cylinders of liquid cargo. U.S. Pat. No. 5,038,960 (Seery) shows a system having a flexible liner in each vessel compartment and a detached interior bladder which hold cargo. U.S. Pat. No. 4,230,061 (Roberts et al.) shows a cargo container extending across the width of a vessel. Multiple containers are arranged end-to-end along the length of the vessel cargo space. U.S. Pat. No. 4,982,678 (Frederick) shows two different types of ship hull liners which are reinforced for tear inhibition and nested one within the other.
An improved system which increases payload possibilities, limits the quantity of potential spills and can be adapted to any of a wide variety of vessel configurations would be a substantial advance in the art.